Gun Digest

CMMG Guard: The Smooth Operator

The CMMG Guard series of pistol-caliber carbines offers a unique operating system that's right on target.

How the CMMG Guard shoots smooth:

CMMG Guard 9

One of the fastest growing competitive shooting segments is the pistol-caliber carbine crowd, which includes events sanctioned by the United States Pistol Shooting Association (USPSA). Known for fast-moving, multi-stage events for race pistols, pistol-caliber carbine matches allow carbines in pistol calibers with a power factor of 125 and a maximum velocity of 1,600 fps. Slings, optical and electronic sights are allowed, as are laser sights — and there’s no restriction on magazine capacity. Because of cost and recoil management, clearly the most popular caliber is 9mm. Besides USPSA sanctioned matches, there are a growing number of carbine-bay matches that include separate classes for pistol-caliber carbines. The growth of competitive shooting opportunities for 9mm carbines, combined with the fact that they’re both fun and cheap to shoot, has spawned the development of a number of new 9mm carbines.

The Struggle With 9mm Carbines

As a result, 9mm might be the second most popular chambering for AR-system rifles. Not long ago in these pages, I reviewed four different PCC guns with varied results. The guns ranged in price from just over $500 for the Just Right JR9C, to more than $2,000 for the Sig MPX carbine. The winner of that test was the SIG MPX because it was the most stable during rapid fire, even though the JR9C was the most accurate.

Stability is important in time/score matches. Most of these events require two shots to neutralize a target. The A zone of the target is 11 inches x 5.875 inches. That’s not a small target, but the matches are scored on speed with penalties for shots outside the A zone. Most distances are under 50 yards, meaning razor-sharp accuracy isn’t as important as speed. The Sig MPX gained an advantage because it was the only gas-operated gun in the test, and gas operation allows for less reciprocating mass … which provides smoother recoil and faster follow-up shots.

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To someone who’s never shot a 9mm carbine in a match where speed is the most important factor, it might be puzzling that we’re talking about recoil from a 9mm carbine. Recoil from the 9mm round isn’t the problem: The problem comes from the reciprocating mass of the semi-automatic operating system.

The Odin Zulu 2.0 adjustable stock adds a few ounces over the original, but the additional weight is next to the center of movement, thus having little effect on speed of transitions. It’s solid and remarkably well made.

Other than the MPX, all the other guns in that four gun test were blowback operated. In a blowback operating system, the mass of the moving slide or bolt carrier provides enough resistance to keep the gun in battery during peak pressure. The movement of that mass, both rearward and forward, disturbs the gun and carries it off the target. The gas operated MPX has a much lighter reciprocating mass and was the smoothest shooting, and therefore it was the fastest in the test.

A properly tuned AR-15 chambered for 5.56 NATO can be tuned to be as stable as a .22 LR carbine. Because a 5.56 AR-15 is gas operated, the bolt carrier can be lightened to reduce the reciprocating mass, reducing bounce. By using the high-velocity gas generated by the 5.56 round, a muzzle brake can be tuned to control the direction of that movement and stabilize the gun.

Unfortunately, in a 9mm carbine, there isn’t enough gas for a muzzle brake to influence the bouncing mass of the system. Because of this greater reciprocating mass and the lack of gas velocity to compensate for movement, accurate double-taps with a blowback-operated 9mm carbine are much harder to accomplish than with a properly tuned 5.56 NATO.

A New Option

This is where the CMMG Guard comes on the stage. CMMG recognized the problem and addressed it by creating a different operating system for their Guard series of 9mm carbines. They accomplished this by redesigning the bolt and bolt recesses in the barrel, and by creating what they call a “rotationally actuated, dual-pin supported linkage, radial delayed blowback operating system” that’s covered by two United States patents.

The rotary bolt on the CMMG Guard series is angled with matching lugs in the barrel. Recoil pressure rotates the bolt and unlocks the carrier, delaying the system and allowing a much lower reciprocating mass of bolt and carrier.

That’s a mouthful, but here’s how it works: When the gun fires, recoil pressure pushes the bolt to the rear and, instead of flats on the bolt lugs, they’re angled. The angled surfaces in the barrel cause matching angles on the bolt lugs to rotate and unlock the system.

The upshot to this complicated technical talk is that the CMMG Guard has a considerably lighter bolt carrier. With less carrier weight, there’s less reciprocating mass, meaning less bounce on firing. Tested beside a tuned gas operated MPX, the Guard wasn’t quite smooth, but it was noticeably more stable than other carbines, such as the JR, JP and Palmetto.

Ultimately, the MPX probably makes the most effective competitive carbine, but with an MSRP of more than $2,000, it still needs modifications to work even reasonably well for competition. The CMMG Guard’s base price is about a third lower, at $1,299. To be competitive with either gun, you’ll have to replace the handguard to reduce muzzle weight for faster transitions, install a better trigger and upgrade the stock.

Changing Of The Guard

As received out of the box, the CMMG Guard was a perfectly workable carbine for normal use. I chose the base model, since the plan was to extensively modify the gun to make it more suitable for competition. If you follow some of my work, you’ll notice that I have developed a recent “family” of parts from specific brands that have come together really well for me … especially when it comes to tweaking for speed. All of these after-market components are great on their own, but when added together on a custom built — things get impressive in a hurry.

The Blackhawk ambidextrous charging handle uses a detent system to maintain closed position, allowing for fast movement without the need to mechanically unlatch.

As delivered, the MkGs T has a 16-inch medium taper barrel, threaded ½-28 for a compensator or suppressor. The trigger of the MkGs T 9MM is standard mil-spec — it was workable, but it was hardly appropriate for competition. The handguard is the short CMMG RKM11, and there’s a standard A2 Pistol Grip with an M4 six-position mil-spec receiver extension buttstock. The billet receivers are 7075-T6 AL upper and lower. The Guard came with a Glock 33 round magazine and weighed 6 pounds.

As modified, the weight remained the same at 6 pounds, but with the improvement of an Isler full-length carbon-fiber handguard slotted for M-Lock accessories. This handguard allows pushing the support hand almost to the end of the barrel, steading the gun during the shot and allowing more leverage to get the gun moving for a transition. The full-length Isler tube actually weighs less than the original short tube.

The standard M4 stock rattles around a bit and doesn’t provide much traction from the plastic buttplate. The much more substantial Odin Zulu 2.0 adjustable stock I chose adds a few ounces over the original, but the additional weight is next to the center of movement, having little effect on speed of transitions. The lighter handguard weight in the front countered the gain at the rear. Ergo’s Tactical Deluxe grip was a huge improvement over the standard A2 hard plastic grip. The larger diameter and “sticky” surface make holding the gun in one hand during reloads easy. Speaking of reloads, I also added a Blackhawk ambidextrous charging handle and a Taran Tactical magazine extension that bumps the magazine capacity up to 41 rounds.

Arguably the biggest change was the trigger. Standard, mil-spec triggers don’t facilitate fast and accurate shooting. A really good trigger allows the shooter to keep the gun stable and allows double taps with low split times. For a trigger, I chose the Elftman AR9 trigger specifically designed for 9mm carbines.

The light weight of a reflex sight allows faster target acquisition than is possible with a much heavier riflescope. The Burris Fastfire 3 has a mount designed for proper head alignment with AR carbines and weighs only a few ounces.

Triggers for 9mm carbines can be an issue. Because of the faster bolt speed and shape of 9mm carbine bolt carriers, most standard AR triggers won’t work in 9mm carbines. The Elftman AR 9 is designed to function with those different shapes, and it works perfectly with no doubles or reset problems. It breaks cleanly at about 3 pounds and makes shooting the Guard a pleasure.

The Guard On The Range

The modifications had little effect on benchrest performance of the unmodified gun. Using Aguila 115-grain 9mm rounds at 50 yards with a Burris Fastfire 3-dot sight, my 10-shot groups measured around 3 inches with a tight cluster in the center, indicating my difficulty with dot sights and astigmatism. Group size would probably shrink with a scope, but scopes add weight and weight is an enemy in fast-transition shooting. I was pleased with that level of performance for the task.

The Burris Fastfire 3 has a 3-MOA dot that automatically compensated for changes in light intensity and turns itself off when not in use. It allows both-eyes-open shooting and fast target acquisition. I chose the 3-MOA dot over the 8-MOA dot because some of the stages I shoot involve 8-inch steel at 50 yards, and I felt the smaller dot would allow more accuracy.

So, the upshot is that the CMMG Guard is a better mousetrap in AR-15 9mm carbines. Maybe it isn’t as good as the best-in-class MPX, but it’s far less expensive and is an AR with familiar AR controls and the ability to easily find and attach almost any accessory you can imagine.

The Elftman AR 9 trigger is designed to work with all AR 9mm carbines, and it provides a precision trigger with no fear of reset and chain-fire issues. I chose the flat-front model.

As delivered, it’s an improvement on every other AR-15 9mm — and when upgraded, it’s a better gun than a basic SIG MPX.

Everything done to the Guard could be accomplished by anyone with a reasonable knowledge of mechanics, and the result is a highly competitive carbine for PCC competition.

For more information on CMMG, please visit

For more information on Aguila Ammo, please visit

Editor's Note: This article originally appeared in the December 2018 issue of Gun Digest the Magazine.

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